Langerhans cells (LC) reside in the epidermis of the skin--a barrier site that provides LC access to cutaneous pathogens and neoplasia as well as benign commensal microorganisms. Work from our group and others indicates that LC suppress immune responses to cutaneous antigen in vivo and may prevent inappropriate responses to skin commensal microorganisms. A key regulated step in the life-cycle of LC is activation and migration from the epidermis to the lymph node where they present antigen to T cells. LC migration occurs in response to a variety of inflammatory stimuli. Though IL- 12 and TNF1 are thought to be key mediators of LC migration, introduction of pharmacological levels of these cytokines in vivo induces migration of only a relatively small number of LC suggesting that other mechanisms of LC activation/migration exist. We have previously shown that LC development requires autocrine TGF21. To examine the role of TGF21 in the steady state, we have recently developed transgenic mice that express tamoxifen inducible Cre selectively in LC. When bred to TGF21-flox mice, tamoxifen injection ablates TGF21 from adult LC and induces spontaneous activation and migration of virtually all LC. Based on this unexpected finding, we hypothesize that autocrine TGF21 signaling plays a key role in LC migration. We propose a model in which TGF21 secreted by LC is activated by integrin av26 on keratinocytes and then acts directly on LC to maintain their immature state. When keratonocytes encounter a danger signal, they decrease surface expression of av26 thereby reducing the amount of activated TGF21 available to LC. The resulting decreased levels of TGF21 signaling in LC promotes their activation and migration. This represents a novel mechanism of DC activation that would have a significant impact on skin and DC biology in general could provide a therapeutic target for the inhibition LC activation. The objective of this proposal is to test the validity of this hypothesis and explore its underlying mechanisms.